Electronically controlled anti-lock systems are commonly included in the standard equipment of medium-class and upper-class automotive vehicles. In the majority of cases, the most important input and control signals of the anti-lock system are produced by means of wheel sensors which sense the rotational behavior of the individual vehicle wheels and send the data to the electronic unit for evaluation. In the so-called controller which includes the electronic unit, braking pressure control signals are calculated which are transmitted to actuators for the modulation of the braking pressure or for braking pressure control. The actuators are typically electrically controllable hydraulic valves.
Identifying the desired control response from the data provided by the wheel sensors and the consistent controlling of the braking pressure for anti-lock control is known to be difficult when the interpretation of the rotational behavior of the wheels does not permit a definite indication of the instantaneous road conditions and the vehicle behavior.
The instantaneous friction value or coefficient of friction between the vehicle tires and the road is an important factor for anti-lock control. This quantity is approximately determined by the electronic unit by measuring and logically combining the rotational behavior of the individual wheels according to predetermined criteria and algorithms. Data which characterizes the coefficient of friction can be produced, for example, by way of the accumulated pressure reduction time during a control phase or instability of a wheel. In specific situations, i.e., in the event of a small difference between drive torque and brake torque (due to cautiously braking on slippery road surfaces, etc.), however, slowly changing wheel courses may occur which involve the risk of a so-called `sneak-away` condition, meaning that all identification criteria for wheel instability are circumvented because a threshold is not reached. The evaluating logic might be `misled` in such situations. Therefore, the behavior of the wheels must be analyzed according to different principles and criteria in order to preclude misbehavior of the ABS control during these "special situations".
An object of the present invention is to provide a method which permits a quick and reliable assessment of the instantaneous friction value or coefficient of friction between the wheel and the road surface in an electronic anti-lock system of this type. More particularly, the objective is to identify or confirm situations with a low coefficient of friction.
The present invention achieves this object by identifying a control operation on a low friction value or checking a low coefficient-of-friction driving situation determined by the electronic unit and also the reacceleration of the non-driven wheels following a control-induced braking pressure reduction on these wheels. Further, it is evaluated as a special driving situation on a low coefficient of friction between the wheel and the road surface if concurrently, in the reacceleration phase, the maximum reacceleration of the two analyzed wheels is below a specific predefined limit value and the maximum filtered acceleration of these wheels is below another specific, equally predefined limit value, and if the duration of the positive variation of the filtered acceleration in this phase exceeds a predetermined interval, and that in this low coefficient-of-friction situation the braking pressure reduction on these wheels is increased in the following instability phase. This increase can be achieved appropriately by an unpulsed pressure reduction (this is a conventional method), provided the control system includes a pulsed actuation of the pressure reducing valves.
The disclosed method uses the pressure reduction so that the non-driven wheels accelerate in an almost unimpeded or unbraked manner and, thus, are in a position to provide reliable information about the actual vehicle speed. This prevents a `sneak-away` condition which would have as a result an incorrect vehicle reference speed, which is, as is known, the reference speed produced by logically combining the individual wheel speed signals.
In case the subsequent reacceleration of the wheel is in excess of the value which is plausible for the low coefficient of friction, the assumption or logical conclusion that this is a situation with a low coefficient of friction is revised, and the wheel pressure on the analyzed (non-driven) wheel when wheel lock occurs is reduced in a pulsed manner as usual.
In a preferred embodiment of the present invention, the limit value of the maximum acceleration is predefined in the order of between 2.5 and 4 g, for example, is set to a value of 3 g approximately. A value ranging between 1.2 and 2.5 g, especially between 1.5 and 1.8 g, may be favorable as the limit value of the filtered maximum acceleration. An interval between 50 milli-sec and 100 milli-sec, in particular an interval of roughly 80 milli-sec, is appropriate as a critical duration of the positive variation of the filtered acceleration, at the exceeding of which a low coefficient-of-friction condition is identified.
Further advantages, features and possible applications of the present invention can be seen in the following description of an embodiment, making reference to the accompanying drawings.